The "normal" microbial flora, particularly of the gastrointestinal tract, is now recognized as possessing a number of properties of importance to the host, beyond the ability to produce overt disease occasionally. Thus, for example, the enteric flora may assist in a number of ways in the maintenance of nutritional balance in the host; and the resident flora of various body surfaces may exert a suppressive effect upon potential invaders. Less well recognized is the fact that the indigenous microflora is involved in the maintenance of host resistance to a wide variety of heterologous infections, not only within the habitat of the normal flora but also systemically.

Experimental suppression of the enteric flora by means of antibiotics has been found to result in decreased resistance to parenteral infections.1 Studies using animals raised in the complete absence of a living flora, ie, in the germ-free state, have indicated that the normal